PROFESSOR STOKES, ON THE DYNAMICAL THEORY OF DIFFRACTION. 39 



ance presented when a pure spectrum is viewed through a pinhole, or narrow slit, which is 

 half covered by a plate of mica, placed on the side at which the blue is seen. At a consider- 

 able angle of diffraction as many as J 5 or 20 bands might be counted. 



In the first experiment the grating was placed with its plane perpendicular to the light 

 which passed through the polarizer, the grooved face being turned from the polarizer. The 

 light observed was that which was diffracted at emergence from the glass. It is only when 

 the eye is placed close to the grating, or when, if the eye be placed a few inches off, the 

 whole of the grating is illuminated, that a large portion of a tail of light can be seen at once. 

 When only a small portion of the grating is illuminated, and the eye is placed at the distance 

 of several inches, as was the case in the experiments, it is only a small portion of a tail which 

 can enter the pupil. The appearance presented is that of a bright spot on the grooved face 

 of the glass. The angle of diffraction in the first experiment was large, 57° 5' by measure- 

 ment. Besides the principal image, or bright spot, a row of images were seen to the left : 

 the regularly transmitted light lay to the right, right and left being estimated with reference 

 to the position of the observer. These images were due to internal diffraction and reflec- 

 tion, as will be better understood further on. They were separated by small angles, depending 

 on the thickness of the glass, but sufficient to allow of one image being observed by itself. 

 The observations were confined to the principal or right-hand image. 



In the portion of a spectrum of the first class which was observed there was a predomi- 

 nance of red light. In most positions of the pointer of the polarizer the diffracted light 

 did not wholly vanish on turning round the analyser, but only passed through a minimum. 

 In passing through the minimum the light rapidly changed colour, being blue at the 

 minimum. This shews that the different colours were polarized in different planes, or perhaps 

 not strictly plane-polarized. Nevertheless, as the intensity of the light at the minimum was 

 evidently very small compared with its intensity at the maximum, and the change of colour 

 was rapid, it is allowable to speak in an approximate way of the plane of polarization of the 

 diffracted light, just as it is allowable to speak of the refractive index of a substance, although 

 there is really a different refractive index for each different kind of light. It was accordingly 

 the angular position of the plane which was the best representative of a plane of polarization 

 that I sought to determine in this and the subsequent experiments. 



In the first experiment the plane of polarization of the diffracted light was determined by 

 six observations for each angle at which the pointer of the polarizer was set. This took a 

 good deal of time, and increased the errors depending on changes in the direction of the light. 

 Accordingly, in a second experiment, I determined the plane of polarization by single obser- 

 vations only, setting the pointer of the polarizer at smaller intervals than before. Both these 

 experiments gave for result that the planes of polarization of the diffracted light were dis- 

 tributed very nearly uniformly. This result already points very decidedly to one of the two 

 hypotheses respecting the direction of vibration. For according to theory the effect of dif- 

 fraction alone would be, greatly to crowd the planes either in one direction or in the other. 

 It seems very likely that the effect of oblique emergence alone should be to crowd the planes 

 in the manner of refraction, that is, towards the perpendicular to the plane of diffraction. 

 If then we adopt Fresnel's hypothesis, the two effects will be opposed, and may very well be 



